Today, films comprising microfibrillated cellulose (MFC), have proven to give excellent barrier properties (see e.g. Aulin et al., Oxygen and oil barrier properties of microfibrillated cellulose films and coatings, Cellulose (2010) 17:559-574, Lavoine et al., Microfibrillated cellulose—Its barrier properties and applications in cellulosic materials: A review, Carbohydrate polymers 90 (2012) 735-764, Kumar et al., Comparison of nano- and microfibrillated cellulose films, Cellulose (2014) 21:3443-3456), whereas the gas barrier properties are very dependent on the moisture or the relative humidity in the surrounding environment. Therefore, it is quite common that MFC films have to be coated with a polymer film to prevent moisture or water vapor to swell and disrupt the MFC film.
The lack of gas barrier properties such as oxygen or air, at high relative humidity has been investigated and described although most of these solutions are expensive and difficult to implement in industrial environment. One route is to modify the MFC or nanocellulose such as disclosed in JP2554589A1 where MFC dispersion was modified with silane coupling agent. The EP2551104A1 teaches the use of MFC and polyvinyl alcohol (PVOH) and/or polyuronic acid with improved barrier properties at higher relative humidity (RH). Another solution is to coat the film with a film that has high water fastness and/or low water vapor transmission rate. The JP2000303386A discloses e.g. latex coated on MFC film, while US2012094047A teaches the use of wood hydrolysates mixed with polysaccharides such as MFC that can be coated with a polyolefin layer. In addition to this chemical modification, the possibility of cross-linking fibrils or fibrils and copolymers has been investigated. This improves water fastness of the films but also water vapor transmission rates. EP2371892A1, EP2371893A1, claims cross-linking MFC with metal ions, glyoxal, glutaraldehyde and/or citric acid, respectively.
However, many of the above solutions require either a post-treatment step or high dosages in order to be effective. Many of the disclosed solutions also limit the production of a MFC film and particularly re-pulping of the same.
There is thus a need to find a simpler solution of producing such films, preferably something that could be used on a paper or paperboard machine, or modified versions thereof, where a subsequent coating or impregnation step can be avoided, since the number of coating stations usually is limited on a paper machine.